1. Field of the Invention
This invention relates to a computerized system for planning the testing of products as part of a manufacturing process. The invention is illustrated for use in planning the testing of semiconductors in the form of integrated circuit (IC) chips.
Semiconductor chips require testing and grading against specifications before they can be sold. The testing of any lot of product will also sort the functional units into separate grades depending on their electrical characteristics. More rigorous or selective testing can then be performed on these grades to produce products which are guaranteed to perform in the more strenuous environment demanded by military or aerospace applications. In addition, certain customers may require special tests to be performed on the product for their specific use.
Production of semiconductors begins in wafer fabrication during which the electronic circuits are grown on thin wafers of silicon in a series of alternating photographic and deposition processes which create hundreds or thousands of identical circuits on each wafer. This process can take from two to upwards to ten weeks depending on the complexity of the process and the number of "layers" involved.
The effectiveness of the wafer fabrication process is dependent on many environmental factors over which the manufacturer has varying degrees of control but which have a definite effect on the functionality and electrical characteristics of the resulting product. In order to eliminate further expensive processing of non-functional circuits, the wafers are probed at this point--i.e. each circuit is powered up, tested and the failures are marked with ink to be later discarded.
After probing, the individual dice are separated, inspected for visual defects and mounted in the individual packages which allow them to communicate with the real world. These packages come in many shapes and sizes from two-lead metal cans to upwards of sixty-four lead dual in-line pin ceramic or plastic packages. Wires finer than a human hair connect the terminals on the dice to the leads on the packages which are then sealed to protect the chip from environmental and mechanical damage.
After this assembly process is complete, the units are tested and graded according to their electrical performance against the range of specifications. The simplest grades are measured at room temperature and are known as commercial. The more difficult grades are those for military and aerospace applications. These must pass strict government standards after being subjected to extreme environmental stress, including continual operation for a week under high temperatures. In addition, the critical characteristics of the higher grade products must not drift out of specification over the whole operating temperature range.
The testing process can be considered as a series of hurdles which the product must pass. Product which fails can be saved to retest against a different set of hurdles, sold as the highest grade for which it has qualified or conceivably sold as a lower grade if no other demand for the product exists.
The process is made especially complex by the large number of different types and grades of product. For example, in one commercial plant, 80 wafer types are assembled into 160 different package configurations which yield over 2500 different end products after testing. The simplest product has 2 grades while the most complex had 88 different grades of standard catalog items and customer specials.
The task for production control is to identify and schedule the correct sequence of tests and an adequate supply of untested material coming out of assembly to support the demand for all of the items. The goal is to accomplish this with minimum inventories and optimal utilization of scarce and expensive computerized test equipment whil maintaining timely and accurate deliveries.
2. Description of the Prior Art
The standard prior art technique for planning the type of production used for semiconductor chips--that is, the technique for specifically determining the requirements for production of the source materials needed in a higher level product--is called Material Requirements Planning (MRP). Such planning is carried out by a computer operating under software control. The computer computations typically are based on a Bill of Materials which defines the source components of any product, the sub-components of such components, and so forth down to the lowest level materials--usually raw materials.
Briefly, Material Requirements Planning performs its planning function by considering the demand for an item and subtracting out any inventories to determine additional production required. This net requirement is then "exploded", that is, multipIied by the appropriate quantities of each component and advanced by the time needed to produce the item. The result is the demand for each component. The process is repeated for each component in turn until the lowest level is reached.